Recently, from the viewpoint of global environmental conservation, the control of exhaust gases has been strengthened on a worldwide scale year after year. As a countermeasure, catalysts for exhaust gas purification have been used in internal combustion engines. Noble metals such as Pt, Au, and Rh have been used as catalyst components in exhaust gas purification catalyst to efficiently remove HC, CO, and NOx in an exhaust gas. However, these noble metals have the drawbacks that the producing countries are limited to specific countries and resources are running out.
Other than noble metals, base metals have been studied as the metal of a catalyst for exhaust gas purification. However, a base metal catalyst for exhaust gas purification has not been put to practical use, due to its lower exhaust gas purification performance activity, specifically, NOx purification performance activity lower than a noble metal catalyst for exhaust gas purification, which comprises a novel metal.
One of the reasons why a base metal catalyst for exhaust gas purification is not used in motor vehicles, for example, gasoline powered vehicles or diesel powered vehicles, is considered as follows: It is not easy to uniformly coat a honeycomb substrate with a base metal-supported catalyst for NOx purification upon production of a catalyst comprising a honeycomb substrate coated with a base metal-supported catalyst for NOx purification in which an active species is supported by a carrier.
There have been various proposals for the technology for coating a honeycomb substrate with a metal-supported catalyst for exhaust gas purification.
For example, Patent Document 1 describes a production method of a catalyst for exhaust gas purification, comprising the step of introducing a wash coat slurry to a honeycomb substrate, in which the wash coat slurry comprises carrier particles having a volume average particle diameter of 20 to 100 μm, a viscosity modifier of an acid such as acetic acid or an inorganic particle dispersant such as a sol, and a solvent, and the wash coat slurry has a viscosity of 5 to 10 mPas at 25° C. Moreover, as a specific example, Patent Document 1 indicates that the slurry was prepared by mixing a noble metal-supported catalyst comprising Pt and Rh supported on a γ-Al2O3 powder carrier, with water and an alumina sol (acetic acid based), and that the slurry was introduced from the end face of a monolith substrate, dried, and fired to obtain a catalyst for exhaust gas purification. It is considered that the slurry was acidified with acetic acid.
Further, Patent Document 2 describes an exhaust gas cleaning device comprising a wall flow-structured filter material having a porous cell partition and a base metal support formed on said cell partition and comprised of a PM oxidation catalyst, which consists of a base metal selected from alkali metals or lanthanoid elements and is capable of oxidizing PM. Moreover, as a specific example, Patent Document 2 indicates a method in which a slurry of a porous oxide powder, such as a γ-Al2O3 powder and a CeO2 powder, together with a binder component such as an alumina sol, and water, is prepared, adhered to a cell partition, thereafter, fired to form a coating layer, the coating layer is allowed to absorb an aqueous solution of a base metal acetate, and the acetate is decomposed so that a base metal such as potassium or cesium is supported to form a base metal-supported portion.